Why Buddhist Practitioners Don't Eat the Five Pungent Roots

In the past, I was wondering why monks and nuns abstain from eating garlic, three kinds of onions and leeks as they are plants. I only learned why from Surangama Sutra, page 230, click here to read more and sharing by Master Sheng Yen. 

Ananda, all beings live if they eat wholesome food and die if they take poison. In their search for Samadhi, they should abstain from eating five kinds of pungent roots (i.e. garlic, the three kinds of onions and leeks); if eaten cooked, they are aphrodisiac and if raw, they cause irritability. Although those who eat them may read the twelve divisions of the Mahayana canon, they drive away seers in the ten directions who abhor the bad odour, and attract hungry ghosts who lick their lips. They are always surrounded by ghosts, and their good fortune will fade away day by day to their own detriment. When these eaters of pungent roots practise Samadhi, none of the Bodhisattvas, seers and good spirits come to protect them, while the mighty king of demons takes advantage of the occasion to appear as a Buddha as if to teach them the Dharma, defaming and breaking the precepts and praising carnality, anger and stupidity; at their death, they will join his retinue, and at the end of their time in his realm, they will fall into the unintermittent hell. Ananda, practisers of Samadhi should never eat these five pungent roots. This is the first step of gradual practice.

What are the basic causes? Ananda, those practisers who wish to enter the state of Samadhi should first observe strictly the rules of pure living to cut lust from the mind by abstaining from meat and wine and by taking cooked, instead of raw food. Ananda, if they do not abstain from carnality and killing, they will never escape from the three worlds of existence. They should consider lust as dangerous as a poisonous snake and a deadly foe. They should begin by strictly observing the Hinayana's four prohibitions for monks and eight for nuns to regulate the body, and then adhere to the Bodhisattva discipline to ensure the non-stirring of mind. If they observe these precepts, they will wipe out forever the karma that leads to birth and killing. If in addition they cease to steal, they will owe nothing to others and will not have debts to repay. Those who keep the rules of pure living in their practice of Samadhi, will be able to see with their own eyes, without the aid of deva sight, all the worlds in the ten directions.